The present invention relates to communications systems, and more particularly, to methods and apparatus for mitigating the effects of disruptive background noise components in communications signals.
Today communications are conducted in a wide variety of potentially disruptive environments, and modern communications solutions are therefore often equipped to compensate for such environments. For example, the microphone in a typical landline or mobile telephone will often pick up not only the voice of the near-end telephone user, but also any surrounding near-end background noise which may be present. This is particularly true in the context of office and automobile handsfree solutions. Since such background noise can be annoying or even intolerable to the far-end user, many of today""s telephones are equipped with noise reduction processors which attempt to suppress the background noise while permitting the speaker""s voice to pass through without distortion. Such noise reduction processors are often based on the well known technique of spectral subtraction in which the spectral content of a noisy speech signal is analyzed, and those frequency components having poor signal-to-noise ratios are attenuated. See, e.g., S. F. Boll, Suppression of Acoustic Noise in Speech using Spectral Subtraction, IEEE Trans. Acoust. Speech and Sig. Proc., 27:113-120, 1979.
When implementing a noise reduction processor, it is important to minimize any artifacts or delay which might be introduced, as such artifacts and delay can be as bothersome to the far-end user as is the background noise.
Accordingly, the above incorporated patent applications disclose spectral subtraction noise reduction systems which introduce low signal distortion as compared to conventional spectral subtraction techniques. Specifically, pending application Ser. No. 09/084,387 discloses a block-based spectral subtraction noise reduction processor in which signal filtering is carried out in the frequency domain using a reduced-variance, reduced-resolution gain function filter. Advantageously, the order of the gain function is chosen such that the frequency-domain filtering corresponds to a true, non-circular convolution in the time domain, and a phase is added to the gain function so that the gain function is causal. As a result, the disclosed noise reduction processor introduces fewer tonal artifacts and fewer inter-block discontinuities as compared to conventional spectral subtraction techniques. Moreover, pending application Ser. No. 09/084,503 discloses techniques for further reducing the variance of the filter gain function and for thereby further reducing the introduction of tonal artifacts. Specifically, the filter gain function is averaged across blocks, for example in dependence upon a measured discrepancy between the spectral density of the noisy speech signal and the spectral density of the noise alone.
While the frequency-domain spectral subtraction filtering techniques of application Ser. Nos. 09/084,387 and 09/084,503 work particularly well in the context of block-based systems (i.e., systems such as the well known Global System for Mobile Communication, or GSM, in which signals are by definition processed sample-block by sample-block), the block-processing times associated with those techniques may not be suitable for applications requiring extremely short signal processor delays. For example, in wire-phone systems, the maximum tolerable signal delay can be as short as 2 ms (corresponding to 16 samples at the standard 8 kHz telephone sampling rate). Consequently, there is a need for improved methods and apparatus for performing noise reduction by spectral subtraction.
The present invention fulfills the above-described and other needs by providing noise reduction techniques in which spectral subtraction filtering is performed in sample-wise fashion in the time domain using a time-domain representation of a spectral subtraction gain function computed in block-wise fashion in the frequency domain. By continuously performing time-domain filtering on a sample by sample basis, the disclosed methods and apparatus can avoid the block-processing delays associated with frequency-domain based spectral subtraction systems. As a result, the disclosed methods and apparatus are particularly well suited for applications requiring very short processing delays. Moreover, since the spectral subtraction gain function is computed in a block-wise fashion in the frequency domain (e.g., using the techniques of the above incorporated co-pending application Ser. Nos. 09/084,387 and 09/084,503), high quality performance in terms of reduced tonal artifacts and low signal distortion is retained. In applications where only stationary, low-energy background noise is present, computational complexity can be reduced by generating a number of separate spectral subtraction gain functions during an initialization period, each gain function being suitable for one of several predefined classes of input signal (e.g., for one of several predetermined signal energy ranges), and thereafter fixing the several gain functions until the input signal characteristics change.
In an exemplary embodiment, a noise reduction processor includes a time-domain filter configured to convolve a noisy input signal with a time-domain spectral subtraction gain function to provide a noise reduced output signal, a spectral subtraction gain function processor configured to compute a frequency-domain spectral subtraction gain function as a function of the noisy input signal, and a transform processor configured to provide the time-domain spectral subtraction gain function by transforming the frequency-domain spectral subtraction gain function. Advantageously, the time-domain filter can continuously convolve the noisy input signal with a prevailing time-domain spectral subtraction gain function, and the prevailing time-domain spectral subtraction gain function can be periodically updated by the transform processor. As a result, the exemplary noise reduction processor can provide extremely short delay times between the noisy input and noise-suppressed output signals. Moreover, samples of the noisy input signal can be delayed prior to being convolved with the time-domain spectral subtraction gain function so that the sound quality of the noise-suppressed output signal can be adjusted. Additionally, a minimum phase can be added to the frequency-domain spectral subtraction gain function to provide a causal time-domain filter having a short delay.
The above-described and other features and advantages of the invention are explained in detail hereinafter with reference to the illustrative examples shown in the accompanying drawings. Those of skill in the art will appreciate that the described embodiments are provided for purposes of illustration and understanding and that numerous equivalent embodiments are contemplated herein.